HEAT. 139 



264. The cause is supposed to be electrical, as the flashes are 

 more brilliant, when the atmosphere is most highly charged 

 with electricity. "In walking in my garden," says Mr. Trimmer, 

 u ia which was a considerable quantity of Nasturtium in bloom, 

 not at all thinking of the flashing of plants, I was struck with 

 the very vivid flashes that proceeded from them ; the scintilla- 

 tions were the most brilliant that I had ever observed, at the 

 same time the sky was overcast with a thunder-cloud ;" and he 

 further remarks, that he always found them most brilliant under 

 such circumstances. The lower orders of plants, as the fungi, 

 have long been noticed as giving light under particular circum- 

 stances. Some in New Holland, species of Agaric, are said to 

 produce light enough to read by. In the mines of Germany 

 certain fungi have been long celebrated for the light they emit. 



Section 2. — Heat. 



265. Heat is the most obviously necessary, of any external 

 agent, to the existence and growth of vegetables ; without a 

 considerable degree of it no vegetation takes place. We observe 

 amid the colds of winter vegetable life is suspended, and as 

 the warmth of spring comes on, vegetation commences, and as 

 the heat increases, plants become more vigorous, in the same 

 proportion. 



266. The beautiful arrangement in the vegetable economy, 

 for the adaptation of vegetables to this season of repose, can but 

 afford matter for the most agreeable contemplation. In equa- 

 torial regions, where heat is constant, a great proportion of the 

 vegetables are of a peculiar organization, not yielding their 

 leaves, not covered with bark, and producing no coverings to the 

 buds ; while in higher latitudes we find our forest trees expressly 

 adapted to a season of repose, or a kind of hybernation. The 

 leaves at the approach of summer come forth in immense pro- 

 fusion, perform with energy their functions during the heat of 

 summer, and at the approach of autumn disengage themselves, 

 by their own depositions, from the parent stock. We find also 

 our forest trees covered with a thick bark, composed of mate- 

 rials possessing the least power for conducting caloric; and the 

 buds, the rudiments for the perfection of which the succeeding 

 year's energies are to be devoted, inclosed in scales, nicely fitted 

 for the protection and preservation of their important contents. 



I A & P t d ^ US 5 \ W „ ha * exam P les a mong the lower order of olants ? 

 ZrX^ ha V S + f. ldof heat? -266. What arrangement does nature make 

 in reterence to this agent at the equator and in high latitudes ? 



